And snow metamorphism

INTERACTIONS BETWEEN SNOW METAMORPHISM AND CLIMATE PHYSICAL AND CHEMICAL ASPECTS... 

The physical impact of the snowpack depends on its physical properties, such as albedo and heat conductivity. Its chemical impact depends on its chemical composition and its reactivity, determined in part by the light flux inside the snowpack. All of these properties change with time, because of a set of physical and chemical processes regrouped under the term snow metamorphism , defined below. 

Snow is a porous medium formed of air, ice crystals and small amounts of chemical impurities. Because ice has a high vapor pressure (165 Pa at -15°C, 610 Pa at 0°C), the vertical temperature gradient that is almost always present within the snowpack generates sublimation and condensation of water vapor that change the size and shape of snow crystals. This results in changes in physical variables such as density, albedo, heat conductivity, permeability and hardness. These physical changes have formed the basis for the definition of snow metamorphism. ... 

Next to the temperature gradient inside the snowpack, an important driving force for snow metamorphism is wind, that lifts, transports and redeposits snow crystals, changing snowpack mass and density " and deposits aerosols inside the snowpack.Wind and temperature are climatic variables that determine metamorphism and snowpack physical properties such as albedo and heat conductivity. These properties affect the energy balance of the snow-atmosphere and of the soil-snow interfaces, which in turn affect climate. 

During a snowfall, the snow crystals accumulate on the ground and gradually form a complex porous medium constituted of air, water vapour, ice and sometimes liquid water. This ground-lying snow transforms with time, depending on the physical parameters of the environment. This process, called metamorphism, can be divided into three main types the wet snow metamorphism, the isothermal metamorphism, and the temperature gradient (TG) metamorphism. 

In this study, to clarify the effect of water vapor, we chose a wide temperature range (-65 °C to -12 °C), for which the water vapor concentrations differed by a factor 1000 between the lowest and highest temperatures, and carried out snow metamorphism experiment under high temperature gradient in a cold room. The density change with water vapor was measured and the effects of water vapor transport on crystal growth, and on density change were examined experimentally. 

Here,/is the water vapor flux, C is the saturated water vapor concentration, De is the water vapor diffusion coefficient in air, T is the temperature, and z is a position in the sample. In this study, it was assumed that the water vapor in the snow sample was saturated. Although there were some arguments about water vapor diffusion coefficient in snow and tortuosity dependence, we did not use those values because we have no data on how those values change during dry snow metamorphism. In reality, we think those values probably affect the water vapor flux. 

Magma molten fluid formed within the Earth s crust or mantle containing molten silicates, water, and gases magma that is extruded on to the surface of the Earth is called lava Metamorphism change created in rocks by heat, pressure, and chemically active fluids within the Earth s crust Meteoric (water) water that percolates rocks from above (e.g. from rivers, rain, snow, etc.)... 

The grain growth almost always observed during metamorphism results in a decrease in snow SSA. The rate of decrease greatly affects snow albedo and e-folding depth, considered over large spatial and temporal scales. The initial decrease is very fast, with a factor of 2 decrease in I to 2 days. Experimental and field studies have quantified the rate of decrease of snow SSA as a function of temperature and temperature gradient." In all cases, the best empirical fit of SSA decay plots was of the form ... 

If fine-grained snow of SSA 200 cm. g is replaced by a melt-freeze crust of SSA 20 cm. gthe reverse would be true and a positive feedback of 3 to 5°C would be produced. From Figure 2a, we predict that more limited positive feedbacks will be produced by a 5°C warming without a change in metamorphic regime, of the order of 1°C. 

Adsorption. The concentration of adsorbed species in snow is determined by snow SSA, temperature and by the partial pressure of the species in snowpack interstitial air. Thus, the reduction in SSA usually observed during metamorphism will lead to the emissions of adsorbed species. Adsorbed species are also readily available for dark and light-induced chemical reactions. ... 

This lead to an extensive work of classification that takes presently the form of an official reference document. Due to metamorphism and packing, any snow layer other than fresh snow is sintered at various levels depending on the snowpack s history. Hence the main problem is to choose a geometric definition of a grain that is consistent with snow physics and mechanics. 

THE TEMPERATURE GRADIENT METAMORPHISM OF SNOW MODEL AND FIRST VALIDATIONS USING X-RAY MICROTOMOGRAPHIC IMAGES... 

Among these different kinds of metamorphisms, the last one is probably the most interesting. Typically occurring by cold and clear nights, when the TG between the top and the bottom of the snow layer is high, this metamorphism is characterized by the formation of facets at the bottom of the grains, while upper parts remain rounded. ... 

Since the TG metamorphism may be the source of weak layer formation in the snow cover, its study has major issues in avalanche sciences, and is an active research field in snow and ice community (see the introduction of Sommerfeld," for a detailed review until 1983). Despite of this interest, the TG metamorphism remains quite poorly understood. In particular, two fundamental questions have not really been solved. First, what is the driving force of the matter exchange in the ice matrix and what are the associated mechanisms Second, what determines practically whether well-rounded or faceted shapes can appear ... 

In a preceding work on snow isothermal metamorphism, we introduced a simple numerical model based on this local equation (I). However, T = T was a constant and P C,T) was just the Kelvin equation. In our present case, we would like to obtain a similar expression, which also takes into account the temperature dependency. By combining the Clausius-Clapeyron and Kelvin equation, we have thus ... 

---